Six hundred eighty HIV-infected infants and children aged 2 to 19 years who require Pneumocystis carinii pneumonia (PCP) prophylaxis will be enrolled in this study nationwide. Subjects will be randomized to one of two arms, trimethoprim-sulfamethoxazole (TMP/SMX) or atovaquone plus azithromycin. Subjects will be evaluated for safety and tolerance of the study drugs. Cross-over to alternative study drug regimen will occur for either arm if a serious treatment-related toxicity or serious bacterial infection endpoint is observed. Patients will be followed for two years. Intensive monitoring of subjects will occur for the first 6 months, during which time the subjects will be monitored on a regular basis for adverse events. The primary objectives of this study are 1) to determine the comparability of azithromycin plus atovaquone to TMP/SMX in the prevention of serious bacterial infections; and, 2) to determine and compare the long-term safety and tolerance of atovaquone plus azithromycin versus TMP/SMX. Subjects currently enrolled will be seen on three month intervals until this study is closed. Enrollment of 3-4 more subjects is anticipated.